It is known to anchor platforms for offshore production of oil and gas at sea bed foundations, via stiff steel pipes which are pretensioned by the buoyancy of the platform. These platforms are usually called "Tension Leg Platforms" and provide substantial advantages in deep sea, e.g. over 300 m. In these depths the costs for a non-floating installation standing on the seabed will be very high. On the other hand, the sea bed installations (template, B.O.P. etc.) for a normal, flexibly anchored semisubmersible production platform will be expensive and more exposed to corrosion and wear, and the riser lines must also be flexible.
Since a submersible platform with a tension leg anchoring system can have a considerable buoyancy, it will not be affected by the prevailing sea heave, and conventional stiff riser lines may be used. Thereby the sea bed installations can be simplified and equipment may be accessibly installed on the deck of the platform.
In prior art TLP-installations, the production platform is placed in position above its foundation and the tendons are lowered in the same manner as a drill string, wherein each string must "target" its connection point in the foundation. This complicate installation procedure normally takes several days and requires good weather with minimal heave and substantial assistance from well anchored, stable barge ships acting as motion stabilizers.